Electroconductive pastes that are obtained by uniformly mixing and dispersing predetermined amounts of metal powders in electrically insulating resins have been conventionally used.
As a metal powder that is mixed and dispersed in such a conductive paste, for example, flat silver particles produced using a wet reduction method have been proposed (see, for example, Patent Document 1).
More specifically, a flat silver powder composed of particles that have a flat shape and are obtainable by a wet reduction method, characterized in that the average major axis of primary particles thereof obtainable by an image analysis of a scanning electron microscopic image (SEM) is 0.1 μm to 1.0 μm, the average thickness of the primary particles is 10 nm to 100 nm, and the coefficient of variation (CV) value of the average major axis is 0.3 or less.
Furthermore, the average particle size (D50) of the flat silver powder obtainable according to a laser diffraction scattering type particle size distribution analysis method has a value within the range of 0.1 μm to 0.8 μm.
Also, the method for producing such a flat silver powder is characterized in that a reducing agent-containing solution including an ascorbic acid-based reducing agent is added to a silver ion-containing solution including silver nitrate, citric acid in an amount of 0.5 mol to 1.0 mol per mol of silver ions, and gelatin in an amount of 20 g to 40 g per mol of silver ions, the reducing agent-containing solution containing the ascorbic acid-based reducing agent in an amount of 0.4 mol to 0.7 mol per mol of silver ions of the silver ion-containing solution.
Furthermore, similarly, flat silver particles produced using a wet reduction method, and a method for producing the same have been proposed (see, for example, Patent Document 2).
More specifically, the flat silver particles are flat silver particles in which, with regard to the crystal structure, the ratio P200/P111 of the peak P200 of plane (200) with respect to the peak P111 of plane (111), which is obtainable by an XRD analysis is 0.3 or less, and with regard to the morphology, the flat silver particles have a triangular contour when viewed planarly, while the flat silver particles have an average particle size of 0.01 to 1 μm and an average thickness of 0.001 to 0.15 μm.
Also, the method for producing such flat silver particles is characterized by heating an aqueous solution including a water-soluble silver compound to 60° C. or higher in a state of having a carboxylic acid, an amine or a thiol in co-existence, and adding reducing agents in sequence to the aqueous solution including a water-soluble silver compound in that state.